As is well known, demands for a volume holographic digital data storage system that can store a large amount of data, such as data for a motion picture film, have been increasing and various types of holographic digital data storage system have been recently developed for realizing high density optical storage capabilities.
The volume holographic digital data storage system allows a signal beam having information therein to interfere with a reference beam to generate an interference pattern therebetween and, then, controls the interference pattern to be stored in a storage medium made of an optical refractive crystal. The optical refractive crystal is a material which may react differently on different amplitudes and phases of the interference pattern.
Various holograms can be recorded in the storage medium by changing an angle of incidence of the reference beam (angular multiplexing) and/or by moving the storage medium to change a recording area (shift multiplexing), so that a great number of holograms of binary data can be stored in the storage medium on a page-by-page basis.
An example of the conventional apparatus for storing and reconstructing holographic data is described in U.S. Pat. No. 6,490,061 B1.
FIG. 1 depicts a block diagram of a conventional apparatus for storing and reconstructing holographic data, which includes a light source 10, a beam splitter 20, two reflection mirrors 30 and 40, a shutter 45, a spatial light modulator (SLM) 50, a storage medium 60, a CCD 70, a data arranging unit 80 and a signal processing unit 90.
The light source 10 generates a laser beam. The beam splitter 20 splits the laser beam into a reference beam and a signal beam and transfers the separated reference and signal beams along two different optical paths, wherein the reference beam and the signal beam correspond to a transmitted beam and a reflected beam, respectively.
The reference beam is reflected at the reflection mirror 30 so that the reflected reference beam is transferred to the storage medium 60. The signal beam, on the other hand, is reflected at the reflection mirror 40 so that the reflected signal beam is transferred to the SLM 50. In a recording mode, the shutter 45 stays open to allow the signal beam to be transferred to the SLM 50.
In the meantime, binary input data to be stored are arranged in a data arrangement of M1 bits in a row and M2 bits in a column on a page basis at the data arranging unit 80, wherein M1 and M2 being positive integers, respectively. Hereinafter, the arranged binary input data consisting of M1×M2 data pixels are referred to as arranged data. The SLM 50 modulates the reflected signal beam with the arranged data transferred from the data arranging unit 80 to provide a modulated signal beam for each page.
The modulated signal beam is transferred to the storage medium 60. The reflection mirror 30 functions to change the reflection angle of the reflected reference beam by a small amount for data storage on a different page.
The interference pattern of the modulated signal beam interfering with the reference beam is stored in the storage medium 60. In this case, the interference pattern stored in the storage medium 60 may be consisted of M1×M2 data pixels and, hereinafter, it is referred to as a stored data image. Typically, the stored data image has “light” and “dark” images, the “light” images representing a logic value “1” and the “dark” images representing a logic value “0”.
During a reconstruction mode, the shutter 45 turns to be closed so that a signal beam cannot be introduced to the storage medium 60. Accordingly, only a reference beam is irradiated onto the storage medium 60.
When a reference beam is irradiated onto the storage medium 60 in order to reconstruct the data written thereon, the reference beam is diffracted by the interference pattern stored in the storage medium 60 so that a reproduced data image for each page may be restored. In this case, the reproduced data image may be consisted of M1×M2 data pixels and it is correspondent to the stored data image.
The reference beam used to reproduce the data written in the storage medium 60 should be irradiated thereon at the same incident angle as that of the reference beam used to record the data.
The reproduced data image is directed toward the CCD 70 to thereby be detected by the CCD 70 and transferred to the signal processing unit 90. The CCD 70 outputs a page image having a size of N1×N2 image pixels, with N1 and N2 being positive integers, respectively. In this case, the page image includes a detected data image corresponding to the reproduced data image.
If each data pixel of the reproduced data image corresponds to P1×P2 image pixels of the page image, the detected data image may be represented by (M1*P1)×(M2*P2) image pixels within the page image, wherein P1 and P2 are positive integers, respectively.
Therefore, image pixels should be sampled from the detected data image at the signal processing unit 90, so that a processed data image is consisted of M1×M2 data pixels that are equivalent to the size of the arranged data.
In a holographic data storing/restoring apparatus, however, the light intensity representing a logic value “1” or “0” within the page image is high at a point in the central region of the P1×P2 image pixels and decreases rather rapidly as the detecting point moves away from the central region toward the peripheral region thereof.
In case that an image pixel placed at the peripheral region of the P1×P2 image pixels is sampled from the detected data image, it is hard to classify the image pixel into one of two binary values, i.e., “0” or “1”, accurately. Accordingly, it is preferred to extract an image pixel placed at the central region of the P1×P2 image pixels in order to classify “dark” images and “light” images into “0” and “1” accurately. In detail, each image pixel at the central region of the P1×P2 image pixels is to be sampled at intervals of P1 and P2 image pixels in a vertical and a horizontal direction, respectively.
However, due to imperfections in a holographic data storing/restoring apparatus, there may be distortions or offsets between the arranged data and the detected data image. As a result, the detected data image can be decreased or increased along a vertical and/or a horizontal direction, so that the detected data image can be consisted of (M1*P1+V)×(M2*P2+H) image pixels, with V and H being integers, respectively. Here, V and H are the number of image pixels representing a resized vertical length and a resized horizontal length of the detected data image, respectively.
In this case, if a conventional sampling method such as above is carried out on the detected data image, there may be image pixels sampled at the peripheral region of the P1×P2 image pixels. In other words, if the detected data image is not consisted of (M1*P1)×(M2*P2) image pixels, the conventional sampling method may have errors.
For this reason, it would be desirable to provide an apparatus for processing holographic data reproduced from a storage medium 60, which is capable of reducing or correcting these errors.